Flying as close as possible to obstacles so as to perform a contour flight (civil or military) or a tactical flight (military) is extremely difficult since the pilot can rely only on what can be seen, on external aids (sensors and cameras), usually without any distance information, and on knowledge about the aircraft being flown (specifically the resources it has available and its load factors).
Under conditions of poor visibility or at night, flights of this type become practically impossible.
At best, the aids to piloting that are presently available provide information for avoiding obstacles and for indicating their presence, but they do not inform the pilot about possibilities of “optimized” flight over obstacles.
The following conventional abbreviations are used herein:
OWS: obstacle warning system;
LIDAR: light detection and ranging;
RADAR: radio detection and ranging;
CCD: charge-coupled device;
LI: light intensification;
IR: infrared;
MMI: man/machine interface;
HUD: head-up display;
HMS/D: helmet mounted sight/display;
IRS: inertial reference system;
INS: inertial navigation system;
AHRS: attitude and heading reference system;
GPS: global positioning system;
FOV: field of view;
FOR: field of regard.
Various automatic piloting systems or systems for assisting the piloting of an aircraft at low altitude have been proposed, that make use of a guard curve (or profile) associated with the aircraft and that compare said curve or profile with the profile of the terrain being overflown and/or of the terrain extending in front of the aircraft.
Such a guard curve can be constituted essentially by a simple horizontal straight line extending at a predetermined height below the aircraft, referred to as the “guard” height; in order to take account of obstacles lying ahead of the aircraft, and in order to delay giving an order to pull-up once such an obstacle has been detected as crossing the horizontal guard line, the curve may comprise a horizontal portion extended by a sloping segment so as to present a “ski toe” shape.
U.S. Pat. No. 3,396,391 describes improvements to a guard curve of this type and proposes modifying the curve as a function of the modulus and of the inclination of the aircraft velocity.
That patent describes in particular a pull-down guard profile comprising a first circular arc of radius corresponding to the pull-up radius, and a second circular arc tangential to the first arc and of radius corresponding to the sum of the pull-up radius plus the pull-down radius, the values of these radii depending on the square of the aircraft velocity; that patent also describes a device that delivers pull-up or pull-down orders as a function of comparing such a guard profile with the profile of the terrain as scanned by a radar.
French patent No. 2 712 251 and U.S. Pat. No. 5,555,175 describe a method of assisting piloting in which a guard curve is used to determine which obstacles are dangerous for the aircraft; in that method, the field in front of the aircraft and that is likely to be reached from the current position is subdivided into angular sectors, and for each sector obstacles are determined as seen by a telemetry detector; thereafter the tops of the obstacles are compared with the guard curve in order to select those obstacles that are closest to the curve; the tops of these obstacles are presented to the pilot superposed on an image of the external landscape together with a hair cross indicating the direction of the velocity vector of the aircraft, either in the form of individual hair crosses or in the form of a smoothed curve interconnecting the tops and referred to as a piloting curve; in order to fly as close as possible to the obstacles, the pilot must superpose the velocity vector hair cross on said curve.
Although that method presents advantages, it is not adapted to pull-down stages of flight; furthermore, that method does not deliver a pull-up order or a pull-down order.
The safety line (or safety cordon) calculated by that method disappears when pulling-up too severely; the pilot is required to maintain the velocity vector of the aircraft above the safety cordon (with an alarm being issued as soon as it goes below), said safety curve preventing the pilot from overflying obstacles as closely as possible.